Contact sensitive device

ABSTRACT

A contact sensitive device includes a member capable of supporting bending waves and a plurality of sensors mounted on the member for measuring bending wave vibration in the member. The sensors measure the bending wave signals and by calculating a phase angle for each measured bending wave signal and a phase difference between the phase angles of least two pairs of sensors so that at least two phase differences are calculated, the location of the contact can be determined.

CROSS-REFERENCE TO RELATED APPLICATION(S)

[0001] This application claims the benefit under 35 U.S.C. § 119(e) ofapplication No. 60/432,024, filed Dec. 10, 2002, which application isincorporated herein by reference.

BACKGROUND

[0002] 1. Field of the Invention

[0003] The invention relates to contact sensitive devices.

[0004] 2. Description of the Related Art

[0005] Visual displays often include some form of touch sensitivescreen. This is becoming more common with the emergence of the nextgeneration of portable multimedia devices such as palm top computers.The most established technology using waves to detect contact is SurfaceAcoustic Wave (SAW), which generates high frequency waves on the surfaceof a glass screen, and their attenuation by the contact of a finger isused to detect the touch location. This technique is “time-of-flight”,where the time for the disturbance to reach one or more sensors is usedto detect the location. Such an approach is possible when the mediumbehaves in a non-dispersive manner i.e. the velocity of the waves doesnot vary significantly over the frequency range of interest.

SUMMARY

[0006] According to one embodiment of the invention, there is provided acontact sensitive device comprising a member capable of supportingbending waves, a plurality (e.g. three or more) sensors mounted on themember for measuring bending wave vibration in the member, whereby eachsensor determines a measured bending wave signal. A processor calculatesa location of a contact on the member from the measured bending wavesignals, in that the processor calculates a phase angle for eachmeasured bending wave signal, then calculates a phase difference betweenthe phase angles of least two pairs of sensors from which the locationof the contact is determined.

[0007] According to another embodiment of the invention, there isprovided a method of determining information relating to a contact on acontact sensitive device that includes providing a member capable ofsupporting bending waves and a plurality of sensors (e.g., three ormore) mounted on the member for measuring bending wave vibration in themember, applying a contact to the member at a location, using eachsensor to determine a measured bending wave signal and calculating thelocation of a contact from the measured bending wave signal bycalculating a phase angle for each measured bending wave signal,calculating a phase difference between the phase angles of at least twopairs of sensors and determining the location of the contact from the atleast two calculated phase differences.

[0008] The following features may be applied to both the device and themethod with the processor being adapted to provide many of thecalculations or processing steps of the method.

[0009] Reflected waves may be suppressed by placing an absorber incontact with the edges of the member. The mechanical impedance of theabsorber and member may be selected so as to significantly reduce orminimize reflections of bending waves from the edges of the member. Inparticular, the impedances may be selected such that bending wave energyis strongly absorbed in a frequency band around a chosen frequency ω₀.The impedance of the absorber may be selected to be both resistive andcompliant. The impedances may be selected to satisfy the followingequation:

Z _(T) =−iZ _(B)(ω₀)

[0010] where Z_(T) is the termination impedance of the absorber, Z_(B)is the mechanical impedance of the edge of the member, and i is theimaginary number={square root}{square root over (−1)}.

[0011] The absorber may be made from foamed plastics which may have openor closed cells and may be polyurethane or polyvinylchloride. Forexample the foam may be a soft PVC predominantly closed cell foam suchas MIERS™ or a medium to high density, open cell polyurethane foam. Theabsorber may extend substantially around the periphery of the member.The absorber may act as a mounting which supports the member in a frameor to another surface.

[0012] The member may comprise a raised pattern on its surface whereby acontact drawn across the surface provides a variable force to the memberto generate bending waves in the member. The pattern may be periodic, orquasi-periodic with a statistically well-defined spatial distribution ofundulations. The pattern may be random whereby a contact travelling overthe surface of the member generates a random bending wave signal. Therandom relief pattern may be an anti-reflective coating, an anti-glaresurface finish, or an etched finish such as those which are found onmany known transparent panels placed in front of electronic displays.

[0013] The member may be a liquid crystal display screen comprisingliquid crystals utilized to excite or sense bending wave vibration inthe member.

[0014] Each measured bending wave signal may be processed by a band-passfilter with a pass-band centered at the chosen frequency ω₀ and having abandwidth of Δω. The bandwidth Δω of the filter is preferably chosen toaddress the Doppler effect whereby a bending wave arrives at a pointwith a different frequency from its original frequency. Accordingly, thebandwidth preferably obeys the relationship:

Δω>>2k(ω₀)ν_(max)

[0015] where ν_(max) is the maximum lateral velocity of the contactacross the surface, e.g. if the contact is provided by a stylus, ν_(max)is the maximum velocity that a user is capable of moving the stylus.

[0016] The phase of each filtered signal may be measured by comparisonwith a reference signal. The reference signal may have a frequency ω₀.The measured phase is the average phase difference between the input andreference signals, optimally measured over the interval 2π/Δω.Alternatively, the reference signal may be derived from a filteredsignal from a second sensor in which case, the measured phase is thephase difference between two input signals.

[0017] The phase differences may be calculated at intervals of 2π/Δωwhich may be intervals of less than 10 ms. The reference and inputsignals may be fed to a phase detector. Output from the phase detectorsmay be fed through low-pass filters having frequency cut-offs ofapproximately Δω/2, then through digitisers and finally through aprocessor to calculate the phase angle θ.

[0018] The instantaneous phases, θ_(l)(t) and θ_(m)(t), of two measuredbending wave signals may satisfy the phase difference equation:

Δθ_(lm)=θ_(l)−θ_(m) =k(ω₀)Δx _(lm)+2πn _(lm)

[0019] where Δx_(lm)=x_(l)−x_(m) (x_(m) and x_(l) being the distancefrom the contact location to each sensor labelled m and l respectively),and k(ω) is the wavevector. This equation may be satisfied if the pathlength difference between two sensors is less than the coherence lengthof the bandpass filter, which is defined as$x_{c} = \frac{2\quad {\pi\omega}_{0}}{\Delta \quad \omega \quad {k( \omega_{0} )}}$

[0020] The coherence condition is therefore |Δx_(lm)|<<x_(c). If thecoherence condition is not satisfied, the above phase equation may notbe satisfied.

[0021] Thus, values of n_(lm) and the phase angle difference arerequired to determine the location of the contact. The shape of themember may be selected to constrain the magnitude of Δx_(lm) to valuesless than half of one wavelength, ie. |Δx_(lm)|<π/k(ω₀). In this case,where all possible values of Δx_(lm) satisfy the condition|Δx_(lm)|<π/k(ω₀), there is only one value of n_(lm) which is theinteger n_(lm) satisfying |Δθ_(lm)−2πn_(lm)|<π. Alternatively, n may beestimated or inferred in some way.

[0022] Another class of foams that has been found to be suitable areacrylic closed cell foams. These may have a high degree of damping andrelatively high stiffness. Such properties are particularly suited toedge termination of stiff, heavy materials such as glass. Examplesinclude 3M serial numbers 4956, 4910, 4950, and 4655.

[0023] Each phase angle difference in combination with the range ofpossible values of the integer n_(lm) may be used to generate a seriesof path length differences thereby defining a series of discretehyperbolic curves on the surface of the member, denoting possiblelocations of the contact. The location of the contact may be determinedby plotting each hyperbola defined by each path length difference andselecting a point at which a large number of the hyperbolae intersect ornearly intersect. This point is likely to be the true location of thecontact.

[0024] Where n_(lm) is unknown, the minimum number of series ofhyperbolae required to determine the contact location is three and thelikelihood of determining the correct location of the contact isincreased by increasing the number of hyperbolae to be plotted. Multiplesensors may be used whereby a phase angle difference may be calculatedfor each pair of sensors thus generating multiple hyperbolae. In thisembodiment, the minimum number of sensors is three.

[0025] Alternatively, where n_(lm) is unknown, the measured bending wavesignal from each sensor may be divided into two or more discretefrequency bands whereby a phase angle difference may be calculated foreach frequency band and for each pair of sensors. Although multiplephase angle differences may be calculated from a single pair of sensors,the phase angle differences at different frequencies are derived fromthe same path length difference. Thus the minimum number of sensors isthree. The dividing of the frequency bands may be achieved by processingthe bending wave signals by at least two band-pass filters havingdifferent pass-band frequencies. For example, using two band-passfilters having frequencies ω₀+ω_(δ) and ω₀−ω_(δ) the phase angledifferences Δθ_(a), Δθ_(b) from two sensors may be defined as

Δθ_(a) =k(ω₀+ω_(δ))Δx+2πn _(a)

Δθ_(b) =k(ω₀−ω_(δ))Δx+2πn _(b)

[0026] where Δx is a single path-length difference defined by thecontact and the position of the sensors.

[0027] Therefore the values of n_(a) and n_(b) may be selected so thatthe measured phase angle differences infer similar values of thepath-length difference. There may be only one combination of values(n_(a), n_(b)) for which this is possible. In this case the true valueof the path-length difference may be determined. The correct combination(n_(a), n_(b)) may be determined as the combination of values thatminimise the expression:${\frac{{\Delta \quad \theta_{a}} - {2\pi \quad n_{a}}}{k( {\omega_{0} + \omega_{\delta}} )} - \frac{{\Delta \quad \theta_{b}} - {2\pi \quad n_{b}}}{k( {\omega_{0} - \omega_{\delta}} )}}$

[0028] The path length difference may then be estimated as:${\Delta \quad x} = {\frac{1}{2}( {\frac{{\Delta \quad \theta_{a}} - {2\pi \quad n_{a}}}{k( {\omega_{0} + \omega_{\delta}} )} + \frac{{\Delta \quad \theta_{b}} - {2\pi \quad n_{b}}}{k( {\omega_{0} - \omega_{\delta}} )}} )}$

[0029] Where this process is repeated with two pairs of sensors, twopath length differences may be determined, which in turn may be used todetermine the location of the contact.

[0030] Alternatively, where n_(lm) is unknown, an initial determinationof the location of the contact may be made using the methods taught inWO01/48684 and PCT/GB2002/003073 (as summarized in FIG. 11). Thereafter,if the condition Δω>>2k(ω₀)ν_(max) holds, the phase angle differenceschange by small increments over the timescale Δt=2π/ω. Accordingly, eachvalue of n may be chosen to minimize the change in path lengthdifference.

[0031] Measured phase angle differences may contain random errors whichmay result in the selection of incorrect values of n. This error may bealleviated by evaluating the likelihood of successive sequences of n,for example by a state-space estimator such as the well known Kalmanfilter. The sequence having the maximum measure of likelihood isselected.

[0032] The state-space estimator provides an estimate of the internalstate of a system of which noisy measurements are made. A necessaryinput to the state-space estimator is a statistical description of theevolution of the system state. An example of such a state is the set ofcoordinates that describes the position and velocity of an object incontact with the member. It is widely known that the Kalman filter andother state-space estimators may provide a measure of likelihood that asequence of observed, noisy measurements are consistent with the modelof the system state.

[0033] A state-space estimator may therefore be employed to take asequence of a pair of path-length differences (say Δx₁₂ and Δx₃₄) takenat different times (say t₁, t₂, t₃, . . . ), to estimate thesystem-state, i.e. the position and velocity of the contact, at thosetimes. Moreover, the overall likelihood of those values of path-lengthdifference being consistent with the model of the system may beevaluated.

[0034] Where the sequence of path-length differences are obtained from asequence of phase-angle differences and a set of integers (n=n(t₁),n(t₂), n(t₃), . . . ), the measure of likelihood generated by thestate-space estimator may be used to infer the likelihood that thecorrect values of n were chosen. It follows that a method for choosingthe correct sequence of integers, n, is to find the sequence for whichthe state-space estimator gives the maximum measure of likelihood.

[0035] As mentioned above, the state space estimator uses somestatistical description of the evolution of the system state. A suitablemodel for the motion of the contact may be a simple random walk.Alternatively, the model may employ a detailed statistical descriptionof how the user moves the stylus or finger. One example is a statisticaldescription of how the user moves a pen while writing text or individualcharacters.

[0036] The processor may further be adapted to include in thedetermination procedure any available information about where thecontact can be expected. For example, if the member is an input devicefor a graphical user interface where the user is presented with a choiceof ‘buttons’ to press, it may be useful to assume that any contact onthe member occurs within the discrete areas corresponding to thebuttons.

[0037] Alternatively, a map of the probability at which a contact islikely to occur and which is based on the expected behaviour of the usermay be used. The device may comprise a software application with agraphical user interface (GUI) which interacts with the operating systemby means of an application program interface (API) in which the API isadapted to generate the probability map. The probability map may bebased on the location, size, and frequency of use of objects presentedby the graphical user interface. The probability map may also be basedon information about the relative likelihood of the various GUI elementsbeing activated.

[0038] The following characteristics may apply to all embodiments of theinvention. The device may comprise means for recording measured bendingwave signals from the or each sensor over time as the contact movesacross the member. The information relating to the contact may becalculated in a central processor. The sensors may be mounted at orspaced from an edge of the member. The sensors may be in the form ofsensing transducers which may convert bending wave vibration into ananalogue input signal.

[0039] The member may be in the form of a plate or panel. The member maybe transparent or alternatively non-transparent, for example having aprinted pattern. The member may have uniform thickness. Alternatively,the member may have a more complex shape, for example a curved surfaceand/or variable thickness.

[0040] The device may be a purely passive sensor with the bending wavevibration and hence the measured bending wave signals being generated byan initial impact or by frictional movement of the contact. The contactmay be in the form of a touch from a finger or from a stylus which maybe in the form of a hand-held pen. The movement of a stylus on themember may generate a continuous signal which is affected by thelocation, pressure and speed of the stylus on the member. The stylus mayhave a flexible tip, e.g. of rubber, which generates bending waves inthe member by applying a variable force thereto. The variable force maybe provided by tip which alternatively adheres to or slips across asurface of the member. As the tip moves across of the member a tensileforce may be created which at a certain threshold, causes any adhesionbetween the tip and the member to break, thus allowing the tip to slipacross the surface. The bending waves may have frequency components inthe ultrasonic region (>20 kHz).

[0041] The member may also be an acoustic radiator and an emittingtransducer may be mounted to the member to excite bending wave vibrationin the member to generate an acoustic output. The frequency band of theaudio signal of the transducer preferably differs from and does notoverlap the frequency band of the measurements from the sensors. Theaudio signal may thus be filtered, for example, the audio band may belimited to frequencies below 20 kHz, and the vibration measurements maybe limited to frequencies above 20 kHz. A sensor may have dualfunctionality and act as the emitting transducer.

[0042] The or each emitting transducer or sensor may be a bendertransducer which is bonded directly to the member, for example apiezoelectric transducer. Alternatively, the or each emitting transduceror sensor may be an inertial transducer which is coupled to the memberat a single point. The inertial transducer may be either electrodynamicor piezoelectric.

[0043] A contact sensitive device according to the invention may beincluded in a mobile phone, a laptop or a personal data assistant. Forexample, the keypad conventionally fitted to a mobile phone may bereplaced by a continuous moulding which is touch sensitive according tothe present invention. In a laptop, the touchpad which functions as amouse controller may be replaced by a continuous moulding which is acontact sensitive device according to the invention. Alternatively, thecontact sensitive device may be a display screen, e.g. a liquid crystaldisplay screen comprising liquid crystals which may be used to excite orsense bending waves. The display screen may present information relatingto the contact.

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] The present invention may be better understood, and its numerousobjects, features, and advantages made apparent to those skilled in theart by referencing the accompanying drawings in which:

[0045]FIG. 1 is a schematic plan view of a touch sensitive deviceaccording to one embodiment of the invention;

[0046]FIG. 2 is a schematic perspective view of the device of FIG. 1;

[0047]FIG. 3 is a schematic side view of a one-dimensional beam;

[0048]FIG. 4a is a graph showing the amplitude of the reflectioncoefficient against frequency (Hz), the amplitude is unitless since itis a ratio;

[0049]FIG. 4b is a graph showing the phase (in radians) of thereflection coefficient against frequency (Hz);

[0050]FIGS. 5a and 5 b are schematic perspective views of alternativetouch sensitive devices;

[0051]FIG. 6 is a flowchart of a method of finding the location of acontact according to an embodiment of the invention;

[0052]FIG. 7a is a schematic block diagram of apparatus used forcalculating phase angles;

[0053]FIG. 7b is a schematic block diagram of apparatus used with thatof FIG. 7a;

[0054]FIGS. 8a to 8 d are plan views of apparatus according to anembodiment of the invention showing the hyperbolae of path lengthdifferences;

[0055]FIG. 9 is a schematic block diagram of alternative apparatus usedfor calculating phase angles;

[0056]FIG. 10 is a flow chart showing an alternative method ofcalculating the location of the contact;

[0057]FIG. 11 is a flow chart showing a method of calculating thelocation of the contact using the dispersion corrected correlationfunction;

[0058]FIG. 11a is a graph of dispersion corrected correlation functionagainst time;

[0059]FIG. 12a is a schematic block diagram of a contact sensitivedevice which also operates as a loudspeaker, and

[0060]FIG. 12b is a method of separating audio signal and measuredbending wave signal in the device of FIG. 12a.

[0061] The use of the same reference symbols in different drawingsindicates similar or identical items.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0062]FIG. 1 shows a contact sensitive device 10 comprising atransparent touch sensitive plate 12 mounted in front of a displaydevice 14. The display device 14 may be in the form of a television, acomputer screen or other visual display device. A stylus 18 in the formof a pen is used for writing text 20 or other matter on the touchsensitive plate 12.

[0063] The transparent touch sensitive plate 12 is a member, e.g. anacoustic device, capable of supporting bending wave vibration. As shownin FIG. 2, four sensors 16 for measuring bending wave vibration in theplate 12 are mounted on the underside thereof. The sensors 16 are in theform of piezoelectric vibration sensors and are mounted one at eachcorner of the plate 12. At least one of the sensors 16 may also act asan emitting transducer for exciting bending wave vibration in the plate.In this way, the device may act as a combined loudspeaker and contactsensitive device.

[0064] In the following applications, U.S. patent application Ser. No.09/746,405, filed Dec. 26, 2000 entitled “Contact Sensitive Device”naming Nicholas P. R. Hill as an inventor, International PublicationNumber WO 01/48684 (International Application Number PCT/GB00/04851) andInternational Application PCT/GB2002/003073, filed Jul. 3, 2002, whichapplications are incorporated herein by reference, contact sensitivedevices and methods of using the same are described. The applicationsdescribe a device that includes a member capable of supporting bendingwave vibration and a sensor mounted on the member for measuring bendingwave vibration in the member and for transmitting a signal to aprocessor whereby information relating to a contact made on a surface ofthe member is calculated from the change in bending wave vibration inthe member created by the contact.

[0065] By bending wave vibration it is meant an excitation, for exampleby the contact, which imparts some out of plane displacement to themember. Many materials bend, some with pure bending with a perfectsquare root dispersion relation and some with a mixture of pure andshear bending. The dispersion relation describes the dependence of thein-plane velocity of the waves on the frequency of the waves.

[0066] Bending waves provide advantages, such as increased robustnessand reduced sensitivity to surface scratches, etc. However, bendingwaves are dispersive i.e. the bending wave velocity, and hence the “timeof flight”, is dependent on frequency. In general, an impulse contains abroad range of component frequencies and thus if the impulse travels ashort distance, high frequency components will arrive first. U.S. patentapplication Ser. No. 09/746,405, International Publication Number WO01/48684 and International Application PCT/GB2002/003073, a correctionto convert the measured bending wave signal to a propagation signal froma non-dispersive wave source may be applied so that techniques used inthe fields of radar and sonar may be applied to detect the location ofthe contact.

[0067] A mounting 22 made of foamed plastics is attached to theunderside of and extends substantially around the periphery of the plate12. The mounting 22 has adhesive surfaces whereby the member may besecurely attached to any surface. The mechanical impedance of themounting and plate are selected so as to minimise reflections of bendingwaves from the plate edges.

[0068] The relationship between mechanical impedance of the mounting andthe plate may be approximated by considering the one dimensional modelshown in FIG. 3. The model comprises a waveguide 34 in the form of abeam which terminates at an edge mounting 36 having a terminationimpedance. An incident wave 38 travelling down the waveguide 34 isreflected by the mounting 36 to form a reflected wave 40. The incidentand reflected waves are plane waves travelling in the directionperpendicular to the edge. Assuming the mounting 36 satisfies thefollowing boundary conditions:

[0069] (i) the termination impedance only couples into the lateralvelocity, i.e. it does not provide any torque resistance; whereby thebending moment is equal to zero at the edge and

[0070] (ii) the ratio of the lateral shear force and the velocity at theedge is equal to the terminal impedance;

[0071] the reflection coefficient at the mounting is given by:${R(\omega)} = \frac{{{- Z_{T}}/{Z_{B}(\omega)}} - i}{{Z_{T}/{Z_{B}(\omega)}} + 1}$

[0072] where Z_(T) is the termination impedance of the mounting andZ_(B) is the mechanical impedance of the end of the waveguide, given by${Z_{B}(\omega)} = {\frac{B\quad {k^{3}(\omega)}}{2\omega}( {1 + i} )}$

[0073] where k(ω), is the wavevector which may be expressed in terms ofthe bending stiffness, B, and mass per unit area, μ, of the panel,$k = {( \frac{\mu}{B} )^{1/4}\sqrt{\omega}}$

[0074] Thus, the reflection coefficient is determined by the ratio ofthe impedances at the end of the waveguide and the mounting.Furthermore, the impedance of the waveguide is proportional to thesquare root of frequency and is both real and reactive in equal weights(i.e. π/4 phase angle). Accordingly, the reflection coefficient islikely to be strongly frequency dependent.

[0075] The reflection coefficient vanishes, i.e. bending wave energy isstrongly absorbed in a frequency band around ω₀, if the followingcondition is satisfied:

Z _(T) =−iZ _(B)(ω₀)

[0076] Thus, the termination impedance of the mounting must have bothreal and imaginary components, or, equivalently, the mounting should beboth resistive and compliant.

[0077] The plate may be, for example, 1 mm thick polycarbonate sheetwhich has mass per unit area, μ=1.196 kg m⁻² and bending stiffness,B=0.38 N m. The equations above can be used to calculate the impedancesof the plate and absorber required to strongly absorb bending waveenergy around the chosen angular frequency ω₀=2π(900 Hz).

[0078] The impedance, per unit width for a 1 mm beam approximation ofthe plate is

Z _(B)(ω₀)=(l+i)33.8 N s m ⁻².

[0079] The properties of the absorber which provide the desiredabsorption are thus: Resistance per unit width,

Re(Z _(T))=Im[Z _(B)(ω₀)]=33.8 N s m ².

[0080] Stiffness per unit width,

−iIm(Z _(T))ω₀ =Re[Z _(B)(ω₀)]ω₀=1.91×10⁵ N m ⁻².

[0081] The reflection coefficient is a unitless complex number. FIGS. 4aand 4 b are graphs showing the amplitude and phase of the reflectioncoefficient R(ω) varying with frequency. The amplitude of the reflectioncoefficient is zero and its phase is reversed for ω₀ approximately equalto 900 Hz.

[0082] In FIGS. 5a and 5 b, the plate 12 has uniform surface roughnessin the form of a raised surface pattern 28,29. The stylus 18 is drawnacross the surface along a path 30 and as it crosses a raised part orline of the pattern it generates bending waves 32 in the member. Thuscontact from the stylus 18 provides a source of bending wave vibrationin the member. In FIG. 5a, the surface pattern 28 is a periodic patternof raised crossed lines and in FIG. 5b, the surface pattern 29 is arandom relief pattern.

[0083] In the embodiments of FIGS. 2, 5a and 5 b, as the contact movesover the rough surface of the member, bending waves radiateisotropically in the member from the point of contact. The displacementof the member at a distance, x, from the point of contact is related tothe displacement at the point of contact by a transfer function, H(ω;x). At distances larger than the wavelength, λ=2π/k(ω), the transferfunction can be approximated as,${{H( {\omega;x} )} = {\frac{A}{\sqrt{k(\omega)}x}^{\quad {k{(\omega)}}x}}},$

[0084] where A is a constant and k(ω), is the wavevector definedpreviously. Although H(ω; x) strictly only applies to bending waves onan infinite plate, since the mounting strongly absorbs bending wavevibrations, the relationship is satisfied. The transfer function showsthat where a source of bending waves emits a purely sinusoidal frequencywith angular frequency, ω₀, the phase difference Δθ₁₂ betweendisplacements at two locations which are at distances, x₁ and x₂, fromthe point of contact for the source is:

exp(iΔθ ₁₂)=exp[ik(ω₀)(x ₁ −x ₂)]

[0085] This implies the following relationship between the phase angledifference, the path length difference Δx=(x₁−x₂) and an integer n₁₂.

Δθ₁₂=θ₁−θ₂ =k(ω₀)Δx₁₂+2πn ₁₂

[0086]FIG. 6 shows the steps in the method for using this equation todetermine the contact location:

[0087] (a) Measure a bending wave signal with each sensor to givemeasured bending wave signals W_(i)(t) and W_(j)(t)

[0088] (b) Calculate the phase angles θ_(i)(t) and θ_(j)(t) of themeasured bending wave signals W_(i)(t) and W_(j)(t),

[0089] (c) Calculate the difference between the two phase anglesθ_(i)(t) and θ_(j)(t)

[0090] (d) Calculate the location of the contact from

k(ω₀)Δx _(ij)=Δθ_(ij)−2πn _(ij)

[0091]FIG. 7a shows a schematic block diagram of a device forcalculating the phase angle θ_(j) of a bending wave signal W_(j)(t)measured by one of the sensors. The signal W_(j)(t) is a random signaland is thus uncorrelated over long time scales. The signal is firstamplified by an amplifier 42 and then processed by an analogue band-passfilter 44 with a pass-band centered at ω₀ and a bandwidth of Δω.

[0092] A moving source of bending waves may demonstrate the Dopplereffect, whereby a bending wave which has frequency coo and is emitted bya source moving at velocity ν towards a point on a member arrives atthat point with a different frequency defined by ω₀ −k(ω₀)ν. The maximumangular frequency shift between bending waves at two different points onthe member is therefore 2k(ω₀)ν_(max), where ν_(max) is the maximumvelocity of the moving source. If the angular frequency shift becomeslarger than the width of the band pass filter, the phase differenceequation above does not hold. Accordingly, the bandwidth Δω of thefilter 44 is set to be greater than this maximum frequency shift andthus obeys the relationship:

Δω>>2k(ω₀)ν_(max)

[0093] After processing by the filter 44, the resulting filtered signalW′_(j)(t) is an amplitude and phase modulated carrier with frequency ω₀and is defined by:

W′ _(j)(t)=A _(i)(t) sin [ω₀ t+θ _(i)(t)]

[0094] where A_(j)(t) and θ_(j)(t) are the amplitude and phase of thesignal. Both fluctuate over a timescale Δt determined by the bandwidthof the filter, namely Δt=2π/Δω. The maximum frequency at whichindependent phase angle measurements may be taken from the output of thebandpass filter is $\frac{1}{\Delta \quad t}.$

[0095] Since a touch sensor typically provides an updated measurement ofthe contact position every 10 ms, the condition for the minimumfrequency of positional measurement is Δt<10 ms.

[0096] The filtered signal W′_(j)(t) is then passed simultaneously totwo analogue phase detectors 46. Such detectors are well known in theart, for example, see p644 of “The Art of Electronics” by Horowitz andHill. Reference signals each having frequency ω₀ but a phase differenceof π/2 are also fed to the two phase detectors. The outputs of the phasedetectors are passed through low-pass filters 48 each having frequencycut-offs of approximately Δω/2. The outputs of the low-pass filters areproportional to cos(θ_(j)) and sin(θ_(j)) respectively. These outputsare then digitized by digitizers 50 and processed by processor 52 togive the phase angle θ_(j).

[0097]FIG. 7b shows how the reference signals used in FIG. 7a may begenerated. A second bending wave signal W_(i)(t) is measured at a secondsensor. The signal is fed through an amplifier 42 and analogue band-passfilter 44 to generate a filtered signal W′_(j)(t). The filtered signalW′_(i)(t) forms the reference signal which is fed directly to one phasedetector 46. The filtered signal is also fed to the second phasedetector 46 via a device which shifts its phase by π/2. The phaseshifted signal is used as the reference signal to the second phasedetector 46.

[0098]FIGS. 8a to 8 d show how the phase angle differences and hence thepath length differences may be used to calculate the location of thecontact. The equation in step (d) of FIG. 6 defines a hyperbolic curvewhich can be overlaid on the plate 12. FIG. 8a shows the threehyperbolic curves 26 which are generated using three different values ofn_(lm) and the calculated phase angle difference for a pair of sensors16 mounted one on each end of the short sides of the plate 12. SimilarlyFIGS. 8b and 8 c show the hyperbolic curves 26 which are generated bythe phase angle difference and different values of n_(lm) for two otherpairs of sensors. FIG. 8d shows all the hyperbolic curves created by thesensors. The contact location 24 is the point of intersection of threehyperbolic curves, one from each pair of sensors. From the contactlocation 24, the correct value of n_(lm) may be inferred.

[0099] A method of inferring n is implemented using the embodiment shownin FIG. 9. The bending wave signal W₁(t) measured by each sensor issimultaneously processed by two band-pass filters 48,54. Two phaseangles, one for each filter, are calculated, for example as described inFIG. 7. The filters 48, 54 have slightly different pass-band frequencieswhereby two phase angle differences, one for each pass-band frequency,are provided by each pair of sensors.

[0100] The phase angle differences Δθ_(a), Δθ_(b) from the sensors maybe defined as

Δθ_(a) =k(ω₀−ω_(δ))Δx+2πn _(a)

Δθ_(b) =k(ω₀−ω_(δ))Δx+2πn _(b)

[0101] where Δx is a single path-length difference defined by thecontact and the position of the sensors.

[0102] The correct combination (n_(a), n_(b)) may be determined as thecombination of values that minimise the expression:${\frac{{\Delta \quad \theta_{a}} - {2\pi \quad n_{a}}}{k( {\omega_{0} + \omega_{\delta}} )} - \frac{{\Delta \quad \theta_{b}} - {2\pi \quad n_{b}}}{k( {\omega_{0} - \omega_{\delta}} )}}$

[0103] The path length difference may then be estimated as:${\Delta \quad x} = {\frac{1}{2}( {\frac{{\Delta \quad \theta_{a}} - {2\pi \quad n_{a}}}{k( {\omega_{0} + \omega_{\delta}} )} + \frac{{\Delta \quad \theta_{b}} - {2\pi \quad n_{b}}}{k( {\omega_{0} - \omega_{\delta}} )}} )}$

[0104] Another pair of sensors may then be used to determine a secondpath length difference. Each path length difference defines a hyperboliccurve on the panel. The intersection point of these two hyperboliccurves is the location of the contact.

[0105] Note that hyperbolae are defined by values of path lengthdifference or Δx. In general, for a given phase-angle difference,several values of Δx are possible (corresponding to different values ofn). The advantage of using two frequencies is that a single value of Δxcan be obtained for each pair of sensors (using the method of minimizingthe expression described above). The determination of the exact value ofΔx, rather than a series of possible values, constrains the location ofthe contact to a single hyperbola, rather than a series of hyperbolae.The location can be determined exactly from the intersection of twohyperbolae, and hence from two pairs of sensors.

[0106]FIG. 10 shows an alternative method for calculating the locationof the contact from the equation above, namely

[0107] i. Measure a pair of bending wave signals W_(i)(t) and W_(j)(t),one signal being measured by a sensor;

[0108] ii. Calculate the dispersion corrected correlation function ofthe two signals using the method described in FIGS. 11 and 11a;

[0109] iii. Calculate the initial position of the contact using thedispersion corrected correlation function, as described in FIGS. 11 and11a;

[0110] iv. Remeasure bending wave signals W_(i)(t) and W_(j)(t);

[0111] v. Calculate the phase angle of each signal—for example asdescribed in FIGS. 7a and 7 b;

[0112] vi. Calculate the difference between the phase angles;

[0113] vii. Select the value of n_(lm) which minimizes the change in thepath length difference;

[0114] viii. Plot the hyperbola defined by

k(w ₀)Δx _(ij)=Δθ_(ij)−2πn _(ij)

[0115] ix. Repeat steps (iv) to (viii), remeasuring the bending wavesignals at regular intervals Δt, for example Δt=2π/Δω.

[0116] At step (viii), a minimum of two hyperbolae from different pairsof sensors are required to determine the position of the contact.Therefore the entire is performed simultaneously for at least two pairsof sensors.

[0117]FIG. 11 shows a method of calculating the dispersion correctedcorrelation function to reveal the difference in path length between thecontact location and the sensors. The method set out below summarizesthe information in PCT/GB2002/003073. The method comprises the followingsteps:

[0118] (a) Measure two bending wave signals W₁(t) and W₂(t);

[0119] (b) Calculate the Fourier transform of W₁(t) and W₂(t) to arriveat Ŵ₁(ω) and Ŵ₂ (ω) and hence the intermediate function Ŵ₁(ω)Ŵ{dot over(₂)}(ω); where Ŵ{dot over (₂)}(ω) is the complex conjugate Fouriertransform, t represents time ω is 2πf where f is frequency.

[0120] (c) Calculate a second intermediate function M(ω) which is afunction of Ŵ(ω)Ŵ{dot over (₂)}(ω)

[0121] (d) and (e) at the same time as performing steps (a) to (c), thefrequency stretching operation f(ω)=ν(μ/B)^(1/4){square root}{squareroot over (ω)} is calculated using the predetermined panel dispersionrelation k=(μ/B)^(1/4){square root}{square root over (ω)}.

[0122] (f) M(ω) and f(ω)=ν(μ/B)^(1/4){square root}{square root over (ω)}are combined to arrive at the dispersion corrected correlation function:${{G(t)} = {\frac{1}{2\pi}{\int_{- \infty}^{+ \infty}{{M\lbrack {f(\omega)} \rbrack}{\exp ( {\quad \omega \quad t} )}\quad {\omega}}}}};$

[0123] (g) the dispersion corrected correlation function is plottedagainst time with a peak occurring at time t₁₂ as shown in FIG. 11a;

[0124] (h) Δx₁₂ is calculated from t₁₂; Δx₁₂ is the path-lengthdifference between the path lengths x₁ and x₂ from the first and secondsensors to the contact.

[0125] (i) Δx₁₂ defines a hyperbolae which may be plotted as in FIG. 7to calculate the location of the contact.

[0126] As with the method of FIG. 10, a minimum of two hyperbolae arerequired to determine the location of the contact. Thus the ways ofgenerating more hyperbolae discussed above apply equally to this method.

[0127] The second intermediate function M(ω) may simply be Ŵ₁(ω)Ŵ{dotover (₂)}(ω) which gives a standard dispersion corrected correlationfunction. Alternatively, M(ω) may be selected from the followingfunctions which all yield phase equivalent functions to the standarddispersion corrected correlation function: $\begin{matrix}{{\text{(a)}\quad {M(\omega)}} = \frac{{{\hat{W}}_{1}(\omega)}{{\hat{W}}_{2}^{*}(\omega)}}{{{{\hat{W}}_{1}(\omega)}{{\hat{W}}_{2}^{*}(\omega)}}}} \\{{\text{(b)}\quad {M(\omega)}} = \frac{{{\hat{W}}_{1}(\omega)}{{\hat{W}}_{2}^{*}(\omega)}}{\sqrt{{{{\hat{W}}_{1}(\omega)}{{\hat{W}}_{2}^{*}(\omega)}}}}}\end{matrix}$

[0128] (c) M(ω)=Ŵ₁(ω)Ŵ{dot over (₂)}(ω)φ└|Ŵ₁(ω)Ŵ{dot over (₂)}(ω)|┘where φ(x) is a real valued function

[0129] (d) M(ω)=Ŵ₁(ω)Ŵ{dot over (₂)}(ω)ψ(ω) where ψ(ω) is a real valuedfunction

[0130] Alternatively, M(ω) may be the function {circumflex over (D)}(ω)which is the Fourier transformation of the correlation function D(t):

D(t)=∫_(−∞) ^(∞) W ₁(t+t′)W ₂(t′)dt′

[0131] The steps are calculate D(t); calculate {circumflex over (D)}(ω)and apply a frequency stretching operation to arrive at the dispersioncorrected correlation function:${G(t)} = {\frac{1}{2\pi}{\int_{- \infty}^{+ \infty}{{\hat{D}\lbrack {f(\omega)} \rbrack}{\exp ( {\quad \omega \quad t} )}\quad {{\omega}.}}}}$

[0132] Alternatively, at step (f) the following dispersion correctedcorrelation function may be calculated:${G(t)} = {\frac{1}{2\pi}{\int_{- \infty}^{+ \infty}{{{\hat{W}}_{1}\lbrack {f(\omega)} \rbrack}{{\hat{W}}_{2}^{*}\lbrack {f(\omega)} \rbrack}{\varphi_{12}\lbrack {f(\omega)} \rbrack}{\exp ( {\quad \omega \quad t} )}\quad {\omega}}}}$

[0133] where${\varphi_{12}^{*}(\omega)} = {{\sum\limits_{j}{{{\hat{W}}_{1,j}(\omega)}{{\hat{W}}_{2,j}^{*}(\omega)}{\exp \lbrack {{- }\quad {k(\omega)}\Delta \quad x_{j}} \rbrack}}}}$

[0134] where {Ŵ_(1,j)(ω)} and {Ŵ{dot over ( )}_(2,j)(ω)} are the Fouriertransformation and complex conjugate Fourier transformation of twomeasured bending wave signals {W_(1,j)(t)} and {W_(2,j)(t)} and {Δx_(j)}is the path-length difference.

[0135] A sensor may act as both the first and second sensor whereby thedispersion corrected correlation function is an autocorrelationfunction. The autocorrelation function may be calculated applying thesame steps for the dispersion corrected correlation function usingW₁(t)=W₂(t).

[0136]FIG. 12a shows a contact sensitive device which also operates as aloudspeaker. FIG. 12b shows a method for partitioning the audio signaland measured signal into two distinct frequency bands so that thecontribution of the audio signal to the processed measured signal issuppressed. The device comprises a member 106 in which bending waves aregenerated by an emitting transducer or actuator 108 and the contact. Theemitting transducer applies an audio signal to the member 106 togenerate an acoustic output. Before being applied to the member, theaudio signal is filtered by a low pass filter 112 which, as shown inFIG. 12b, removes the audio signal above a threshold frequency f₀.

[0137] As shown in FIG. 12b, the contact generates a signal which has apower output which is substantially constant over a large frequencyband. The signal from the contact and the audio signal sum to give acombined signal which is passed through a high pass filter 114 to removethe signal above the threshold frequency f₀. The filtered signal is thenpassed to a digitizer 116 and onto a processor 118.

What is claimed is:
 1. A contact sensitive device comprising: a membercapable of supporting bending waves; a plurality of sensors mounted onthe member for measuring bending wave vibration in the member, whereineach of the sensors determines a measured bending wave signal; and aprocessor responsive to the measured bending wave signals to calculate alocation of a contact on the member; the processor calculating a phaseangle for each measured bending wave signal and a phase differencebetween the phase angles of least two pairs of sensors so that at leasttwo phase differences are calculated from which the location of thecontact is determined.
 2. A contact sensitive device according to claim1 comprising an absorber at edges of the member whereby reflected wavesare suppressed.
 3. A contact sensitive device according to claim 2,wherein mechanical impedance of the absorber and the member are selectedso as to reduce reflections of bending waves from the edges of themember.
 4. A contact sensitive device according to claim 3, wherein theimpedances are selected so that bending wave energy is strongly absorbedin a frequency band around a chosen frequency ω₀.
 5. A contact sensitivedevice according to claim 4, wherein the impedances are selected tosatisfy the following equation: Z _(T) =−iZ _(B)(ω₀) where Z_(T) is thetermination impedance of the absorber and Z_(B) is the mechanicalimpedance of the edge of the member.
 6. A contact sensitive deviceaccording to claim 4, comprising a band-pass filter for filtering eachmeasured bending wave signal, the filter having a pass-band centered atthe chosen frequency ω₀ and a bandwidth of Δω.
 7. A contact sensitivedevice according to claim 6, wherein the bandwidth Δω of the filterobeys the relationship: Δω>>2k(ω₀)ν_(max) where ν_(max) is the maximumlateral velocity of the contact.
 8. A contact sensitive device accordingto claim 2 wherein the absorber is made from foamed plastics.
 9. Acontact sensitive device according to claim 1, wherein the membercomprises a raised pattern on its surface whereby a contact drawn acrossthe surface provides a force to the member to generate bending waves inthe member.
 10. A contact sensitive device according to claim 9, whereinthe pattern is random whereby a contact traveling over the surface ofthe member generates a random bending wave signal.
 11. A contactsensitive device according to claim 10, wherein the pattern is formedfrom an anti-reflective coating, an anti-glare surface finish, or anetched finish.
 12. A contact sensitive device according to claim 1,further comprising at least two band-pass filters which have differentpass-band frequencies and which simultaneously process the bending wavesignals measured by a pair of sensors whereby a phase angle differencefor each pass-band frequency is provided by the pair of sensors.
 13. Acontact sensitive device according to claim 1, comprising four sensorson the member.
 14. A contact sensitive device according to claim 1comprising three or more sensors on the member.
 15. A contact sensitivedevice according to claim 1, comprising means for determining an initiallocation of the contact using a dispersion corrected correlationfunction of pairs of measured bending wave signals and means fordetermining subsequent locations of the contact using the phase angledifference between pairs of measured bending wave signals.
 16. A contactsensitive device according to claim 1, wherein an initial location ofthe contact is determined using a dispersion corrected correlationfunction of pairs of measured bending wave signals and whereinsubsequent locations of the contact are determined using a phase angledifference between pairs of measured bending wave signals.
 17. A contactsensitive device according to claim 1, further comprising a phasedetector for determining the phase angle.
 18. A contact sensitive deviceaccording to claim 17, wherein the processor comprises a low-pass filterand a digitizer for determining the phase angles.
 19. A contactsensitive device according to claim 1, wherein the member is an acousticradiator and an emitting transducer is mounted to the member to excitebending wave vibration in the member to generate an acoustic output. 20.A contact sensitive device according to claim 19, comprising means forensuring that the acoustic output and measured bending wave signals arein discrete frequency bands.
 21. A contact sensitive device according toclaim 19, further comprising one or more filters to separate theacoustic output from the measured bending wave signals.
 22. A contactsensitive device according to claim 1, further comprising an emittingtransducer mounted to the member to excite bending wave vibration in themember.
 23. A contact sensitive device according to claim 22, whereinthe emitting transducer also functions as one of the sensors.
 24. Acontact sensitive device according to claim 1 wherein the member istransparent.
 25. A contact sensitive device according to claim 1 whereinthe member is a liquid crystal display screen comprising liquid crystalsutilized to excite or sense bending wave vibration in the member.
 26. Amethod of determining information relating to a contact on a contactsensitive device comprising: providing a member capable of supportingbending waves and a plurality of sensors mounted on the member formeasuring bending wave vibration in the member; applying a contact tothe member at a location, using each of the sensors to determine ameasured bending wave signal; and calculating the location of a contactfrom the measured bending wave signals by calculating a phase angle foreach measured bending wave signal, calculating a phase differencebetween the phase angles of at least two pairs of sensors anddetermining the location of the contact from the at least two calculatedphase differences.
 27. A method according to claim 26, wherein theplurality of sensors is three or more.
 28. A method according to claim26, comprising suppressing reflected waves by placing an absorber at theedges of the member.
 29. A method according to claim 28, comprisingselecting the mechanical impedances of the absorber and the member so asto reduce reflections of bending waves from the edges of the member. 30.A method according to claim 29, comprising selecting the impedances sothat bending wave energy is strongly absorbed in a frequency band arounda chosen frequency ω₀.
 31. A method according to claim 30, comprisingselecting the impedances to satisfy the following equation Z _(T) =−iZ_(B)(ω₀) where Z_(T) is the impedance of the absorber and Z_(B) is theimpedance of the edge of the member.
 32. A method according to claim 30,comprising filtering each measured bending wave signal by a band-passfilter having a pass-band centered at the chosen frequency ω₀ and abandwidth of Δ_(ω.)
 33. A method according to claim 26, comprisingapplying the phase difference equation: Δθ_(lm)=θ_(l)−θ_(m) =k(ω₀)Δx_(lm)+2πn _(lm) to determine the location of the contact, where θ_(i) isthe phase angle of a measured bending wave signal, x_(i) is the distancefrom the contact location to each sensor, Δx_(lm)=x_(l)−x_(m) is thepath length difference of two sensors, k(ω) is the wavevector and n_(lm)is an unknown integer.
 34. A method according to claim 33, comprisingselecting the member to constrain the magnitude of Δx_(lm) to valuesless than one half of a wavelength so that n_(lm) is determined from|Δθ_(ml)−2πn_(lm)|<π.
 35. A method according to claim 33, comprisingdetermining an initial location of the contact using the dispersioncorrected correlation function of a pair of measured bending wavesignals and selecting a value of n_(lm) which minimizes change in thepath length difference.
 36. A method according to claim 33, comprisingselecting a series of values of n_(lm), combining the series of valueswith each phase angle difference to define a series of path lengthdifferences, plotting the series of graphs of the path lengthdifferences, and inferring the true value of n_(lm) from a point atwhich a large number of the graphs intersect.
 37. A method according toclaim 26, comprising calculating multiple phase angle differences fromthe pairs of phase angles, plotting a graph of each path lengthdifference and selecting a point at which a large number of thehyperbolae intersect to be the location of the contact.
 38. A methodaccording to claim 26, comprising dividing the measured bending wavesignals from each sensor into at least two discrete frequency bands andcalculating a phase angle difference for a pair of sensors for eachfrequency band.
 39. A contact sensitive device comprising: a membercapable of supporting bending waves; a plurality of sensors mounted onthe member for measuring bending wave vibration in the member; and meansfor calculating the location of a contact from the measured bending wavesignal by calculating a phase angle for each measured bending wavesignal, calculating a phase difference between the phase angles of atleast two pairs of sensors and determining the location of the contactfrom the at least two calculated phase differences.